It is known from the above mentioned application and patents to coat elongated metal members with fusible synthetic-resin material by heating the metal member, bringing it into contact with a fluidized or gas-entrained stream of synthetic-resin particles, thereby bonding the particles in a layer to the metal member, and thereafter causing the synthetic-resin of the particles to flow and coalesce in the formation of a film on the surface of the metal member.
It is known, further, to provide apparatus for externally coating endless metal tubing with synthetic-resin materials by this so-called powder-film coating process, by blowing a turbulent air stream through a vessel containing the power and drawing the elongated metal tubing therethrough after previously heating the metallic surface.
For the continuous coating of round articles such as wire or tubing of small diameter, Austrian Patent 238,349 describes a process in which the round articles are moved through a fluidization bed in a so-called "dip-coating basin" whereupon the adherent layer of powder is reheated to level the synthetic-resin coating. The articles are then cooled in a liquid bath to solidify the synthetic-resin layer.
In practice it has been found that this process cannot be used for a uniform coating of tubes which are relatively large in diameter, e.g. of a diameter of 10 mm or more, because dead zones or wakes are formed on the underside of the tube so that only a smaller amount of the synthetic-resin material can be fritted thereto in these regions. The term "fritted" is used to refer the adhesion of discrete particles of the synthetic-resin to the metallic surface without coalescence of the synthetic-resin.
Austrian patent 310,430 describes a process in which this disadvantage apparently can be eliminated by rotating the tubes to be coated about their axes as they move through the fluidized bed in which they are dip-coated.
Endless tubing, i.e. tubing which fabricated in any conventional plant for the manufacture of metal tubing, cannot be coated by this process because the tube cannot readily be rotated.
Swiss patent 466,103 describes a process whereby a trickling bed, which is vibrated, is used to permit coating of endless tubing which cannot be rotated. However, this bed cannot be used as a practical matter to apply uniform thin coatings to a tube which travels at high speeds since this requires powder having such a small particle size, e.g. 30 to 80 microns, that trickling no longer occurs and is not possible.
Still another process, as described in Austrian patent 323,860, enables an external coating of endless tubing but permits tubing to be coated only when it moves at a velocity below 10 meters per minute. When the tube moves at a higher velocity, the powder-entraining air stream discharged by the annular nozzle and guided onto the tube by a guide vane, sucks additional air from the exit opening for the tube to be coated. This exit opening is disposed beneath the guide vanes so that only a small part of the powder particles contact the heated surface of the tube. Most of the particles are entrained into the annular air exhaust duct. This can be compensated only by providing a distance between the tube inlet and tube outlet in the coating head which is extremely long. Because of the vibration of the tubing to be coated, however, such lengths are not practical and it is found that when they are utilized, vibration of the tubing results in direct contact between the tubing surface coated with the molten material and the surface of the tube or the edge of the tube outlet.